Description
We report on the development of discharge scenarios that enable long-term stabilization ($t > \tau_{L/R}$) of equilibrated high-performance plasmas in the optimized stellarator W7-X. The scenarios exploit repetitive pellet injection combined with second-harmonic O-mode heating, which together may raise the ion temperature beyond the ion-temperature clamping limit (>1.5 keV) and sustain a quasi-stationary operational cycle at high density and plasma energy up to the available heating duration.
Surprisingly, these plasmas exhibit indications of central fueling and the plasma ions appear to contribute to the thermalisation of the injected pellet material. Targeted impurity seeding mitigates heat loads on critical components without impurity accumulation thereby enabling longer pulses. As a consequence of pellet injection, the plasma parameters oscillate within a limited range. In particular, under constant and optimized pellet fueling and plasma heating, good confinement is achieved over several dozen pellet cycles. This PULSE (Pellet-Utilized Long-Pulse Steady-State Enabled) regime proves robust: if irregularities in pellet size temporarily reduce plasma performance, recovery to high performance is observed once the nominal fueling rate is restored.
In initial experiments, central ion temperatures exceeding 2 keV and central densities above $1.5\times10^{20} m^{−3}$ were obtained at the maximum of the pellet-induced cycles. During these phases, central beta values of up to 4 % were achieved, together with confinement consistent with the ISS04 scaling ($f_{ren}≈1$). The improved confinement appears to be related to a local reduction of transport that remains stabilized over a pellet cycle in the PULSE regime. For the first time, dimensionless parameters of reactor relevance have been achieved in stellarator plasmas sustained for up to 43 s.